A conventional cellular network comprises a plurality of base stations, each base station transmitting to one or more mobile terminals (each referred to as a respective User Equipment or UE) using at least one respective carrier. In particular, the Third Generation Partnership Project (3GPP) have specified Long Term Evolution (LTE) architectures, in which each base station (called an eNodeB or eNB in this context) transmits downlink signals to UEs using one or more Orthogonal Frequency Division Multiplexed (OFDM) carriers. Each carrier occupies a frequency bandwidth and therefore defines a data rate capacity for delivering services to the UEs.
Carrier aggregation allows a UE to be served by multiple different carriers at the same time, allowing flexible allocation of data transmission to those carriers. This may increase the data rate that can be communicated between the network and the UE, for instance allowing a UE to reach a high peak data rate by being provided with access to more spectrum. Although originally only standardised for aggregation between two carriers on the same eNB (intra-eNB carrier aggregation), in 3GPP Release 12, aggregation between two carriers on different eNBs is being standardised (inter-eNB carrier aggregation). The specification work is intended to define the way and amount of information to be exchanged (signalling mechanisms) so that aggregation is successfully achieved. Inter-eNB carrier aggregation is most relevant between a macro cell and a small cell and it would typically be used where the coverage areas of the base stations intersect.
Referring first to FIG. 1, there is shown a schematic diagram of a part of a network architecture to permit inter-eNB carrier aggregation. A Master eNB 20 and a Secondary eNB 30 each communicate with a UE 60 using a respective Uu (air) interface 65. An Mobility Management Entity (MME) 70 interfaces with a Home Subscriber Server (HSS) 80 and the MME 70 communicates with the Master eNB 20 and the Secondary eNB 30 over respective S1 interfaces 75. Moreover, the Master eNB 20 and the Secondary eNB 30 can communicate with each other over an Xn interface 50. The Xn interface allows the Master eNB 20 and the Secondary eNB 30 to exchange information and for the Master eNB 20 to control the Secondary eNB 30 if required. The Secondary eNB 30 may also make autonomous decisions. Typically (although not necessarily), the Master eNB 20 is a macro cell and the Secondary eNB 30 is a small cell. One generalised approach for inter-eNB carrier aggregation is discussed in WO-2012/136256. Thus, a carrier may be aggregated at the UE 60, such that Master Base Station (MeNB) 20 transmits carrier frequency X to the UE and Secondary Base Station (SeNB) 30 transmits carrier frequency Y to the UE. The additional, aggregated carrier is referred to as a Secondary Cell (SCell).
3GPP Technical Report (TR) 36.842 v12.0.0 discusses particular techniques for implementing inter-eNB carrier aggregation. Referring to FIGS. 2A and 2B, there are illustrated techniques discussed in this document that have been standardised by 3GPP based on the network architecture shown in FIG. 1. Where the same features are indicated as in FIG. 1, identical reference numerals have been used.
FIG. 2A shows a first technique (referred to as “Alternative 1A” in 3GPP TR 36.842 v12.0.0). Service data 10 is provided to the Master eNB 20 and the Secondary eNB 30 from the MME 70 over the respective S1 interfaces 75. This service data 10 comprises two bearers (for example, voice and data respectively). The bearers are split: one bearer is communicated to the UE 60 through the Master eNB 20; and the other bearer is communicated to the UE 60 through the Secondary eNB 30. Each eNB has respective Packet Data Convergence Protocol (PDCP), Radio Link Control (RLC) and Media Access Control (MAC) layers for processing the bearers.
FIG. 2B depicts a second technique (referred to as “Alternative 3C” in 3GPP TR 36.842 v12.0.0). In this architecture, a single bearer of the service data 10 can be split between the Master eNB 20 and the Secondary eNB 30. In the illustration shown, the Master eNB 20 receives two bearers and one bearer is split at the PDCP layer between it and the Secondary eNB 30. The portion of the bearer for transmission by the Secondary eNB 30 is communicated from the Master eNB 20 to the Secondary eNB 30 over the Xn interface 50. This portion of the bearer is then processed at the RLC and MAC layers by the Secondary eNB 30 for transmission to the UE 60.
Some approaches exist to address a number of issues relating to Inter-eNB carrier aggregation. For example, WO-2013/143051 discusses power saving techniques for inter-eNB carrier aggregation and US-2014/0078989 discusses how inter-eNB carrier aggregation can be set up. Nevertheless, many significant challenges still remain in the implementation of inter-eNB carrier aggregation, such as when to implement it, which implementation technique to use and how to manage resources between the base stations.